“Satellite navigation system” here refers to any system dedicated to wide area navigation, such as for example the existing GNSS (Global Navigation Satellite System) systems, called GPS, GLONASS, or the future GALILEO system, as well as all their equivalents and derivatives. Those skilled in the art know well the principle of location used by satellite navigation systems. The radio signal transmitted by a satellite is encoded and the time taken by this signal to reach the receiver to be located is used for determining the distance between that satellite and that receiver, preferably called the pseudo-distance. The accuracy of satellite navigation systems is affected by a certain number of errors. These errors can be divided into two categories: global contributions and local contributions. For global contributions, it is possible to mention the errors related to the passage of electromagnetic waver in the ionosphere and the errors related to the satellites (orbit and clock errors). For local contributions, it is possible to mention the errors related to the passage of electromagnetic waves in the troposphere, signal reflection errors, the errors related to interference, the errors due to white zones and the noise of the receivers. In order to improve the existing satellite systems in terms of accuracy, integrity, continuity and availability, systems have been provided, known in the spatial field as “augmentation systems”. The European satellite augmentation system EGNOS improves the performance of the two satellite systems GPS and GLONASS. It transmits pseudo-distance corrections in order to correct the abovementioned errors.
The invention is intended to correct ionospheric errors more particularly. It is recalled that the density of the air which constitutes the atmosphere decreases as the distance from the surface of the Earth increases. At ionospheric altitude, the cosmic and solar rays are no longer filtered. These rays (ultraviolet and X) are more aggressive and tear electrons from the atoms constituting the air, which is called ionization. The refraction index is therefore altered and therefore has the variation of the speed of propagation of the signals passing through the ionospheric layer as a consequence. Given that the delay is calculated by assuming a speed of propagation of the navigation signals equal to the speed of light, the passage through the ionospheric layer gives rise to an erroneous pseudo-distance measurement due to a delay in the measurement of the navigation codes or to an acceleration in the phase measurements. In order to obtain better accuracy in the calculation of positions, it is necessary to estimate the ionospheric error which can moreover vary throughout the day.
FIG. 1 describes a satellite navigation system architecture, known to those skilled in the art, comprising a GNSS positioning system and an SBAS (Satellite Based Augmentation System) augmentation system. The aircraft 4 have on-board receivers connected to the EGNOS augmentation systems. EGNOS is a system of the SBAS type comprising, in a ground segment 300, an infrastructure constituted by a plurality of ground stations “SBAS G” and, in a spatial segment 100, a plurality of geostationary satellites “SBAS S”. The ground segment comprises a plurality of ground stations spread over a wide geographic area, which receive data from the GNSS satellites and determine the pseudo-distances, and a central control and processing station 1 which, from the pseudo-distances transmitted by the SBAS G receiving stations, determines the corrections which are grouped in a signal 10. The geostationary satellites “SBAS S” relay this signal 10 from the central station 1 to the receivers of the aircraft 4.
The calculating station 1 compiles the ionospheric corrections data in order to calculate a grid 96 of ionospheric corrections 91 to 94 as shown in FIG. 2. When the navigation signals pass through the ionospheric layer the ground stations “SBAS G” determine piercing points corresponding to the satellite to ground stations line of sight. The whole of the territory covered by the ground stations is thus sampled by piercing points to which correspond the ionospheric delay measurements. When a point is adjacent to points of the ionospheric corrections grid, the value of the ionospheric delay 95 is determined by extrapolating the values 91 to 94 of the adjacent piercing points. The coverage and the fineness of the grid 96 of ionospheric corrections are proportional to those of the network of ground stations and satellites.
These conventional systems (GNSS and SBAS) exhibit several problems. A first problem is the coverage of the area. In fact, the coverage of the grid of ionospheric corrections depends on the distribution of the ground stations which can be deployed only over terrestrial areas of easy access. Outside of these limits, such as for example over maritime or mountainous zones, the navigation systems exhibit a performance degradation. A second problem is the number of samples for calculating the corrections grid. The more delay measurements data there are, the more accurate are the corrections. This number of measurements is directly related to the number of satellites and to the number of ground stations whose cost is high (also because of the maintenance and the real time data communication streams). A third problem is the quality of the measurements. The measurements carried out by the ground stations are affected by local error contributions such as reflection problems, interference and the troposphere for example.
In the prior art there is known the U.S. Pat. No. 6,674,398B2 which describes an invention using mobile receivers for measuring the ionospheric delays. These measurements are then sent directly to the spatial segment which relays these data to a calculating system in the ground segment for compiling them and calculating a grid of ionospheric corrections. However, this invention necessitates creating and having the availability of a dedicated user link to the spatial segment, and leaves the possibility of using by non-aeronautical users and is therefore not worthy of confidence for an aeronautical service.